1. Field of The Invention
This invention relates to optical information processing and, more particularly, to recursive filter means for use in an optical spatial filter system.
2. Discussion Of The Prior Art
It is known in the prior art to use an optical spatial filter and Fourier transform system to perform selective spatial frequency filtering. Such systems usually incorporate a source of collimated coherent light and, in the light path therefrom, modulation is introduced by means of a suitable transducer, which is followed by a convex lens, an optical spatial filter, which can be a programmable spatial filter (PSF), a second convex lens, and detection means. In such a system, light from the source passing through the transducer is modulated and thus forms a diffraction pattern (Fourier transform) at the back focal plane of the first lens. The second lens provides the inverse Fourier transform of the diffraction pattern that passes through the spatial filter, resulting in a spatial-frequency filtered representation of the modulated input light signal. This light signal exiting the second lens contains all the spatial frequencies appearing in the Fourier plane minus those selectively filtered out by the PSF. Thus, the prior art teaches that changing the spatial frequency distribution of the PSF in the Fourier plane results in an inverse transform image with a corresponding resolution/high frequency, contrast/low spatial frequency and phase response.
Adaptive filtering or the electronically programming of a filter in accordance with time varying criteria has been addressed in the prior art. The concept of a filter matched to the spectrum of the desired signal theoretically provides the optimum filtering. In actual (real world) situations, particularly when spread spectrum signals contaminated with higher level narrow band signals are to be detected, a narrow band (notch) filter within the passband of the wide band spread spectrum signal is desired to reduce or eliminated the high level signal.
There are significant advantages to accomplishing this filtering function optically instead of by use of the more usual electronic filter, particularly if a number of filters are desired. The use of optical techniques for high speed adaptive filtering and reconstruction of broadband RF signals is taught in the prior art, for example, by John N. Lee et al in a IEEE Ultrasonics Conference (1981) paper entitled "High-Speed Adaptive Filtering and Reconstruction Of Broad-Band Signals Using Acousto-Optic Techniques", and further in the following:
T. M. Turpin, "Spectrum Analysis Using Optical Processing", Proceedings of the IEEE, Vol. 69, No. 1, Jan. 1981.
W. T. Phodes, "Acousto-Optic Signal Processing: Convolution And Correlation", Proceedings of IEEE, Vol. 69, No. 1 Jan. 1981.
A. Korpel, "Acousto-Optics-A Review Of Fundamentals", Proceedings of IEEE, Vol. 69, No. 1, Jan., 1981. All of these prior art teachings of optical spatial filtering are seen to be directed to a single stage filtering system. In these prior art systems, particularly those using a PSF, the maximum filter attenuation is about 25 dB, which is the maximum attenuation realizable with presently available components with a single stage PSF. In the present invention, since the PSF stages are effectively cascaded by means of recursions, the attenuation can be increased as a function of the number of stages (recursions). The prior art, however, is devoid of any teaching of other than single stage spatial filtering means.